The electrocardiogram (ECG) is a widely used clinical tool for cardiac physiological monitoring and for the real-time diagnosis of heart conditions. Surface ECG monitoring, in which the ECG electrodes are attached to a patient's skin, is one type of ECG monitoring that is often used with medical imaging processes. Frequently, patients with cardiac conditions will undergo, for example, a magnetic resonance imaging (MRI) scan in which ECG information is monitored during the scan sequence and used to assist in MRI image acquisition. The ECG information that is obtained may be used, for example, for properly synchronizing cardiac MRI scans, which is a process typically referred to as “gating.” Because cardiovascular anatomy is continuously moving throughout the MRI scan, synchronization or “gating” of MRI data acquisition with the cardiac cycle allows for improved imaging of the cardiovascular anatomy at the various phases of the cardiac cycle. Surface ECG monitoring is also frequently used for physiological monitoring of patients who are being scanned for multiple indications (brain, knee, abdomen, etc.) or who are undergoing therapeutic interventions inside the MRI (even interventions not within the cardiovascular system). This is especially true for patients who are anesthetized during the scan or who have a history of heart disease or of stroke.
Surface ECG monitoring inside an MRI scanner presents several challenges that can affect the quality, and thus the usability, of the ECG signals and MRI data acquired. First, the ECG electrodes can experience unwanted voltages that are induced during the ramping up and down of the magnetic field gradients used in the MRI scan. Due to the changing magnetic field caused by the gradient rampings, current is induced in the ECG electrodes, with amplitudes up to a few volts. These induced voltages can be many times larger than the voltage levels of the true ECG signals and, therefore, can saturate the ECG acquisition equipment, making the true ECG signals more difficult to accurately filter out and detect. And, the higher voltages can potentially damage the ECG acquisition equipment, which is configured to ordinarily detect only measurements in the range of a few milli-volts. Additionally, the induced voltages can have a frequency content of anywhere between about 100-10,000 Hz, which can render it very difficult to remove the unwanted induced signals with simple frequency filters. Similarly, the ECG electrodes and leads may conduct radio frequency (RF) fields, such as 64 MHz for 1.5T or 127 MHz for 3T, which are induced into the ECG electrodes and leads by the MRI scan sequence's RF pulses. These RF-induced signals cause further noise and, thus, also deteriorate the quality of the signals acquired by the ECG electrodes. Not only do these phenomena negatively affect the acquisition of ECG information, they can interfere with acquiring the desired imaging data.
Attempts have been made to address the issues confronting use of surface ECG monitoring inside an MRI scanner, but have met with only limited success. Most attempted solutions have simply taken the approach of using a smaller number of ECG electrodes (typically around 3 to 5 electrodes) that are closely distanced from each other in order to, in theory, reduce the induced RF and gradient voltages. However, 12-lead ECG surface monitoring arrangements, in which electrodes are placed at designated positions on the torso, is the preferred and most widely used system for monitoring heart condition. Reducing the number of electrodes to only 3 to 5, and arranging the electrodes much more closely than the standard 12-lead arrangement, causes the quality and usefulness of the ECG signals to be severely deteriorated. As a result, such approaches are useful only to perform MRI scan synchronization and do not provide physiological monitoring-quality ECG traces.
Other attempted solutions have involved the use of software filtering or digital signal processing of the acquired ECG signals to remove or suppress the RF and gradient induced components in the ECG leads, but these have met with limited success and also do not provide physiological monitoring-quality traces. Noise caused by gradient ramping and RF transmission during an MRI scan sequence is intrinsically a difficult problem to address with software and digital signal processing, because the gradient noise component in ECG leads is on the order of a thousand times stronger than the true ECG signal component. In addition, because the gradient fields applied by MR systems constantly change to a significant degree in terms of magnitude, direction, frequency, and duration (due to different requirements of each imaging sequence), it is difficult or impossible for signal-processing algorithms to adapt to the large variety of potential gradient noise.
Electro-Anatomic-Mapping (EAM) is a relatively newer clinical tool than traditional surface ECG monitoring, in which ECG data is collected at various positions inside the body, including on the walls of the cardiac chambers. One of the distinguishing differences from surface ECG monitoring is that positional information is also acquired on the same conductive lines as the ones that transfer the ECG signals from the electrodes to the receiver. This positional information is acquired, for example, by inducing electrical currents from surface electrodes and sampling them using catheters that have multiple electrodes on their shaft and that are moved inside the body. The positional tracking signals are generally electromagnetic signals at higher frequencies (5-10 kHz) then those found in conventional ECG (0-300 Hz), so they are easily separated by the EAM receiver. The common practice is to display the position of the catheter electrodes and the ECG voltage at those points, which defines the EAM map. Available EAM systems include the NavX™ systems offered by St. Jude Medical, Inc. and the Carto systems offered by Biosense Webster, Inc.
However, like surface ECG monitoring, EAM mapping inside an MRI scanner also presents several issues that can affect signal quality. First, the ECG component of EAM signals encounters the same RF and gradient induced noise problems as discussed above. Second, the positional localization component of the EAM signals suffer from the induced voltages caused by the gradient ramps as well, since the noise created by the gradient ramps is within the same reception band (5-10 kHz) of the localization voltages. For example, one type of NavX™ system operates using a 5.8 kHz signal, while others operate with 8.1 kHz signals.
These issues confronting the use of EAM inside an MRI scanner have either not been addressed at all, or have only been addressed by an unsatisfactory solution. In fact, the only techniques currently in accepted use that offer positional information in MRI scanners are either based on passive tracking (i.e., using the MRI images themselves for following interventional devices) or use MRI techniques for active tracking (e.g., MR-tracking or MRI-gradient tracking). These solutions do not work outside the MRI scanner, so they cannot be used to monitor a patient during transfer in and out of the scanner, during portions of the procedure which are conducted outside the MRI bore, or during periods in which the patient is inside the scanner, but no images are being acquired.
Moreover, the issues confronting acquisition of ECG and EAM information during an MRI scan can also negatively impact the use of other physiological monitoring tools, such as pulse oximeters, blood pressure cuffs, and respiratory monitors. Each of these clinical tools may include the use of electrodes and leads that can also be susceptible to the same unwanted currents and voltages induced by gradient ramping and RF transmission occurring during an MRI scan.
It would therefore be desirable to provide a system and method for reducing or avoiding the negative effects of RF and gradient induced voltages on ECG and EAM signals (and other physiological monitoring signals), to allow for acquisition of physiological monitoring-quality ECG and/or EAM signals from inside an MRI scanner during a scan. Similarly, it would be further desirable if the system and method allowed for acquisition of such signals both inside and outside the scanner and during periods in which images are being acquired and not being acquired.